The data capacity in the network has been increasing year by year due to the wide spread of the Internet. In the trunk line connecting metropolitan areas, the optical transmission link whose transmission capacity per one channel is 10 Gb/s or 40 Gb/s has already been introduced. On-Off-Keying (OOK) is employed as a modulation scheme in 10 Gb/s transmission. Although the OOK scheme is used also in 40 Gb/s transmission systems, it is unsuitable for long-haul transmission because the transmission characteristics are greatly influenced by the chromatic dispersion due to the narrow optical pulse width of 25 ps. Therefore, a multilevel modulation scheme using phase modulation and a polarization multiplexing scheme have been adopted, and a Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) scheme is mainly employed for 100 Gb/s class transmission systems.
A light signal modulated by using DP-QPSK scheme in a transmitter is received and demodulated by a coherent optical receiver (refer to non patent literature 1, for example). An example of the configuration of a related coherent optical receiver is shown in FIG. 18. The related coherent optical receiver 700 includes a local light source 710, a 90° hybrid circuit (90° HYBRID) 720, optoelectronic converters (O/E) 730, analog to digital converters (ADC) 740, and a digital signal processing unit (DSP) 750.
Signal light and local light can be expressed as single polarization signals by the following formulae respectively.S(t)=exp[jωt]  (1)L(t)=exp[j(ω+Δω)t]  (2)where Δω represents a frequency offset between the signal light and the local light. The signal light and the local light are inputted to the 90° hybrid circuit (90° HYBRID) 720, and after passing through an optical interference system, they are converted into electrical signals by the optoelectronic converters (O/E) 730 which are made up of a differentially configured set of photodiodes. At that time, outputs expressed by the following formulae (3) and (4) are obtained from port IX and port QX respectively.IX(t)=cos(Δωt)  (3)QX(t)=sin(Δωt)  (4)
In a case of a polarization multiplexed signal, S(t) is expressed as S(t)=EX+EY, and a cosine component of the mixed signal EX+EY is outputted at IX and IY ports and a sine component of the mixed signal EX+EY is outputted at QX and QY ports.
The signals outputted from the respective ports are inputted into the digital signal processing unit (DSP) 750, after being AD converted by the analog to digital converters (ADC) 740. In the digital signal processing unit (DSP) 750, EX and EY signals are separated by a polarization demultiplexing process, and then demodulated in 4-levels by a phase estimation process.
In this way, DP-QPSK signals can be demodulated using a coherent optical receiver.    Non Patent Literature 1: M. G. Taylor, “Coherent Detection Method Using DSP for Demodulation of Signal and Subsequent Equalization of Propagation Impairments”, IEEE Photonics Technology Letters, vol. 16, No. 2, February 2004, pp 674-676.